1.1.7 Carbon-Dioxide (CO2) Laser
Lasing action in a CO2 molecule was first demonstrated by C. Patel in
1964.He transmitted an electric discharge pulse through pure CO2 gas in a
laser tube, and got a small laser output. CO2 is the gas in which the lasing
process occurs, but other gas additives to the laser tube improve the total
efficiency of the laser. The standard CO2 laser includes in the active
medium a mixture of CO2 with N2 and He. The optimal proportion of these
3 gases in the mixture depends on the laser system and the excitation
mechanism. In general, for a continuous wave laser the proportions are:
CO 2:N2:He - 1:1:8
CO2 is a linear molecule, and the three atoms are situated on a straight
line with the Carbon atom in the middle. In figure 1.6 the three vibrational
modes of CO 2 molecule are illustrated:
1. Symmetric stretch mode (1).
2. Bending mode (2).
3. Asymmetric stretch mode (3).
Figure 1.6: Oscillation Modes of CO2 Molecule
Lasing transitions in CO2 laser
Lasing transitions in CO2 laser occur when the molecule is going from
higher energy level of the asymmetric mode into one of the other two, as
can be seen in figure 1.7.
1. The transition to the symmetric stretching mode corresponds to the
wavelength 10.6 m.
2. The transition to the bending mode corresponds to the wavelength 9.6
m.
Figure 1.7: Energy Level Diagram of CO2 Laser
Each of the vibrational energy level is subdivided into many rotational
levels. Transitions can occur between vibrational energy levels with
different rotational levels, so there are many lasing lines around the main
vibrational transitions.
CO2 laser operation
Electrical discharge is created in the laser tube. The energy of the
accelerated electrons is transferred by collisions to the Nitrogen molecules
and to the CO2 molecules. Nitrogen molecules help in the process of the
excitation of the CO2 molecules.
The first vibrational energy level of the Nitrogen molecule is very similar
to the asymmetric stretching mode of the CO2 molecule, so energy can be
easily transferred from the excited Nitrogen molecules to the CO2
molecules. Helium molecules are added to the gas mixture in order to:
1. Empty the lower laser energy level so that population inversion is
maintained.
2. Stabilize the electrical discharge by taking heat away from the lasing
area.
Types of CO 2 Lasers:
There are many types of CO2 lasers, all based on the same physical
principles. The differences between them are in their structure, excitation
mechanism, and the output radiation. A few CO2 lasers are described
below.
a. Flowing CO2 gas lasers
In these lasers a fresh gas mixture is flowing continuously through the
laser tube while lasing lasts. Flowing gas is used when the maximum power
is needed out of the CO2 laser. The gas flows along the tube and is released
out into the atmosphere (since it is non poisonous). These lasers are very
simple, and the requirements from the gas purity are small. Hundreds of
Watts can be achieved at the output of these lasers.
b.Sealed off CO2 laser
The gas laser is filled with the appropriate mixture of gasses and sealed
(as we saw in He-Ne laser). High electric voltage is applied to electrodes
at both ends of the gas tube. The accelerated electrons excite the gas
molecules.
The problem with sealed off lasers is the dissociation of the CO2
molecules into CO and Oxygen with time. To reduce this effect, a
catalyzing agent is added to the gas mixture. This catalyzing agent reverses
the dissociation reaction and restores the CO2 molecule which is required
for lasing. Sealed off CO2 lasers are usually limited to output power of less
than 200 watts. For higher output power it is necessary to take away the
heat generated inside the laser, and a flowing gas is needed.
Sometimes a gas reservoir is added to the sealed off tube to allow some
refreshment of the lasing gasses from the reservoir to the tube. The new
class of sealed CO2 lasers is made from metal with no glass tube.
c.CO2 Laser inside a waveguide
When the laser tube diameter is reduced to a size of about 1 millimeter,
a waveguide is made. The radiation inside the small diameter tube is
confined to move along the tube, with low loss. Using ceramic tubes, very
small CO2 lasers can be made. These small CO2 lasers can produce up to
50 Watts of continuous wave radiation.
Transverse Flow CO2 gas lasers
When the gas flow inside the laser is perpendicular to the laser axis, it
is possible to have a much larger flow since the distance is very short. Since
cooling by gas flow is very efficient, it is possible to get very high power
output out of these lasers.
Both the gas flow and the electric discharge in these lasers are along the
width of the laser. Thus, the distance between the electrodes is short, so
electric discharge can be achieved even for gas at very high pressure (up to
a few atmospheres).Transverse flow is used for very high power CO2
lasers.
Properties of CO2 Laser
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High output power.Commercial CO2 Lasers produce more than 10000
Watts continuously.
Output spectrum is in the Infra-Red (IR) spectrum: 9.6 m , 10.6m
Very high efficiency (up to 35%).
Can operate both continuously or pulsed.
Average output power is 75 W/m for slow flow of gas, and up to few
hundreds W/m for fast gas flow.
Very simple to operate, and the gasses are non-toxic.
CO Laser
This laser is very similar to the CO2 laser, except for the active gas CO. The spectrum output of these lasers is: 5-6 m. One of the problems
with this laser is the gas CO which is poisonous.